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Zhang H H, Yuan Y S, Li Z W, Xie C F, Zhang Q D, Chen G R, Wu L, Zhu L D, Pan J T, Li S Z, Hu G Y, Zhang L Y. The discovery of the Early Cretaceous olive basaltic porphyrite in the Wumengshan area on the western margin of the Yangtze block and its implications for the intracontinental response to the continental margin collision orogenic event. Geological Bulletin of China, 2025, 44(2/3): 477−492. DOI: 10.12097/gbc.2023.01.021
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The absence of Yanshanian magmatic activities in the western margin of the Yangtze block has led to a lack of precise time records of tectonic movements during this period. In this study, Early Cretaceous basic intrusive rocks were discovered for the first time in the Wumengshan area of northeastern Yunnan. This discovery is of great significance for exploring the Yanshanian tectonic activities in the western margin of the Yangtze block.
This paper conducts research on the newly discovered intrusive rocks through detailed field geological observations, microscopic identification, zircon U−Pb dating, and petrogeochemical methods, and explores their genesis and geological significance.
The lithology of the Early Cretaceous basic intrusive rocks is olivine basaltic porphyrite, which has a porphyritic texture and amygdaloidal structure. The phenocrysts are mainly plagioclase and olivine. The U−Pb concordia age of 31 zircon measurement points is 134.0 ± 0.4 Ma, indicating that the formation age is the early stage of the Early Cretaceous. Petrogeochemical characteristics show that these rocks are rich in alkalis, high in Ti and Al, and low in SiO2. They have a relatively high total rare earth content, depleted heavy rare earths, and obvious fractionation between light and heavy rare earths elements. These characteristics are highly similar to those of the Permian Emeishan basalts in the study area, suggesting that they are the same origin. They may be the metasomatized and enriched mantle at the tail of the Emeishan mantle plume formed in the Permian. Under the long−range extrusion effect of the subduction−collision in the Bangonghu−Nujiang suture zone, partial melting of the asthenosphere occurred. During the ascending and emplacement process, they underwent mild contamination with the crust, forming basic−ultrabasic intrusive rocks.
The Early Cretaceous olivine basaltic porphyrite indicates that the continental−margin collision orogeny between the Tethys tectonic domain and the western margin of the Yangtze Block has a significant long−range effect. Its discovery has extended the research on Yanshanian magmatic activities in western Yunnan to the Wumengshan area east of the Xiaojiang Fault Zone, filling the gap in the study of Yanshanian magmatic activities around the Sichuan Basin. The formation of olivine basaltic porphyrite also indicates that the large−scale intra−continental compressional orogeny in the western margin of the Yangtze Plate reached its peak. 134 Ma is the time record when the temperature and pressure of the intra−continental orogeny in the western margin of the Yangtze block reached their peak.
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